In wireless communication systems, a terminal (such as a mobile telephone or a fixed wireless access terminal) communicates with one of a number of base stations. However, the terminal need not always communicate with the same base station, and may switch base stations even during a call. The decision as to which base station to communicate with at any particular time will depend on the channel quality between each base station and the terminal. The channel quality will vary with each base station because of a number of geographic effects. For each base station, the geographic effects include: the distance between the terminal and the base station, which will affect the received signal strength; the number of intervening obstacles and terrain features between the terminal and the base station, resulting in shadowing; and reflections off nearby obstacles, resulting in multipath fading. The channel quality will also be affected by interference from other channels. The channel quality for each base station often varies with time as the interference varies. Variations in the channel quality may also occur due to varying geographic effects, particularly with mobile terminals for which the communication path between the terminal and each base station will vary quite quickly.
In order to determine the optimum base station for communication with a terminal, a base station controller (BSC) keeps track of a candidate set of base stations for each terminal. A candidate set is a list of base stations for which a channel quality sufficient for communication with the terminal has been measured for communication between the base station and the terminal. For each base station within communication range, the terminal determines the channel quality by measuring the strength of a pilot signal from the base station and reports the channel quality to the BSC via the base station. The BSC communicates with all of the base stations, and collects the channel qualities reported by the terminal for each base station. The BSC stores the identities of the base stations for which the channel quality is above a threshold necessary for adequate communication with the terminal. The base station identities form a candidate set for the terminal. The candidate set will vary with time, as the channel qualities change due to movement of the terminal or due to changing interference levels.
During a call, the BSC also maintains an active set for each terminal. An active set is a list of one or more base stations with which an active terminal is actually communicating (an active set may list more than one base station identity if the terminal communicates with more than one base station simultaneously, as in communication systems that employ Code Division Multiple Access). The initial active set for a terminal is determined by the BSC based upon the traffic load on each base station within the candidate set of the terminal, in an attempt to balance the loads on all base stations. If the channel quality between the terminal and a base station in the active set falls below a threshold (such as occurs when the terminal moves beyond the range of the base station) the BSC determines a new active set, and transmits the new active set to the terminal via the base stations.
As described above, the determination of the candidate set for a terminal is determined by the BSC based on the strength of the pilot signals received at the terminal from the various base stations. The strength of each pilot signal gives a measurement of the channel quality of the respective downlink channel. Alternatively, the candidate set can be determined on the basis of the channel quality of each uplink channel. A base station may determine the channel quality of the uplink channel by measuring the quality of the Random Access Channel when a terminal initiates a call, or by measuring the quality of the System Access Channel during a call. The base station reports the channel quality of the uplink channel to the BSC, which then updates the candidate set for the terminal.
A single candidate set is therefore maintained for each terminal, based either on the channel quality of the uplink channel or the channel quality of the downlink channel. However, since the uplink and the downlink use different channels, it is possible for a particular base station to be the optimum base station for traffic in one direction but not in the other direction. For voice traffic this is somewhat acceptable. Voice traffic is generally symmetric in that the volume of traffic in the uplink direction and in the downlink direction are comparable. This means that about 50% of the traffic will be using the optimum base station. However, as wireless communication systems evolve to become more data oriented, the volume of traffic will tend to become asymmetric. For example, handheld web browsers typically transfer much more traffic over the downlink than over the uplink. For an individual call, if the volume of traffic over the downlink is much greater than the volume of traffic over the uplink, it would be preferable to determine the optimum base station (or base stations) based on the channel qualities of the respective downlink channels. Similarly, if the volume of traffic over the uplink is much greater than the volume of traffic over the downlink, it would be preferable to determine the optimum base station (or base stations) based on the channel qualities of the respective uplink channels.